Increased demands on the running qualities of the gears in gear drives require more and more the precision finish machining of the gear teeth. In most cases the pre-machined workpieces are case-hardened, and then fine machined to the finished dimensions on all their functional surfaces. In this process the fine machining of the tooth flanks in particular is a complicated, resource consuming, and hence expensive operation. In the interests of economic manufacture, but also in order to avoid having to provide for unnecessarily large hardening depths and to balance and minimize the wear on the left and right flanks of the fine machining tool, it is therefore attempted to keep the material allowance for the fine machining as small as possible. For the fine machining of the teeth this means in practice that the depth of cut per flank for material removal is only a few hundredths to at most two tenths of a millimetre. If, as is generally the case, the left and the right flanks are machined in the same operation, this demands a very accurate alignment of the precut teeth relative to the fine machining tool, so that the latter can be brought exactly into the centre of the tooth spaces to be machined, in order to achieve uniform material removal from the left and right flanks of the workpiece.
Another requirement to be fulfilled by the aligning device is that it can be adapted to the relevant workpieces to be machined over a wide diameter range and axial position of the workpiece teeth to be measured.
Whereas for the alignment of the mutually related positions of tool and workpiece teeth to be machined the NC-axes available on the finishing machine are usually exploited, various methods are encountered in practice for the registering of the position of the precut workpiece teeth and the translation of the measured result into appropriate axis motion commands for the machine axes. In most cases, as also here in the present case, the angular position of the precut workpiece teeth is measured by means of a non-contact functioning measuring probe based on an inductive, optical or magnetic principle. Pre-requisite for an adequately exact and reliable measurement is that for the measurement the measuring probe is located near the outer contour of the teeth of the rotating workpiece in an exact tangential and axial position relative to the workpiece. During the subsequent machining of the workpiece, or whilst the work spindle is being loaded with a new workpiece, the measuring probe should, however, as far as possible be located outside the machining area at a point at which it is adequately protected against collision and soiling with swarf and grinding dust.
The solving of this seemingly trivial problem is not easy because, due to the short process times demanded, the measuring probe must be run to the measuring position at high speed and with high accuracy, must remain in position without vibrating during the measuring process, and then return just as quickly to its protected starting position.
With the known aligning devices with retractable measuring probe the probe is brought by means of a linear infeed or a swivel action about a fixed axis against a stop, or by means of a combination of both out of its position of rest into its measuring position, and from there back into its starting position. The motions are produced by hydraulic, pneumatic or electric motor drives.
Necessitated by the usually tight space conditions in the working area of the machine, solutions are often encountered in which by means of long projecting slender structural parts, long displacement strokes and wide swivel angles great spacial distances must be overcome. The results of this are mostly a modest stiffness of the measuring structure and a high proneness to vibration, as well as to functional disturbances, and accuracy loss due to soiling and wear under severe production conditions. The aligning errors caused by this lead to unequal material removal on the left and right flanks of the workpiece, and to rejects as soon as after finish machining—due to the small machining allowance—individual flanks are no longer machined over the entire flank surface.